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Actinolite schist is a medium-grade metamorphic rock characterized by a significant proportion of the amphibole mineral actinolite, which gives the rock its distinctive green color. It exhibits a well-developed schistosity, meaning the platy or elongated mineral grains (primarily actinolite, chlorite, and mica) are aligned parallel to each other, creating a foliated texture. Other common minerals include epidote, albite, quartz, and sometimes garnet or biotite. The rock's appearance can range from dark green to grayish-green, often with a silky or fibrous sheen due to the actinolite.
How to Identify
- Color
- Typically light to dark green, grayish-green, or sometimes blackish-green.
- Luster
- Often silky or vitreous on cleavage surfaces, dull on fractured surfaces.
- Texture
- Strongly foliated (schistose) with visible, elongated, often needle-like or fibrous actinolite crystals aligned parallel. Can feel slightly greasy or soapy due to chlorite.
- Crystal Form
- Actinolite crystals are typically acicular (needle-like), prismatic, or fibrous. In schist, they are usually elongated and aligned.
- Cleavage
- Actinolite itself has two perfect cleavages at approximately 56 and 124. The schistosity of the rock represents a pervasive planar fabric, not individual mineral cleavage.
- Geological Environment
- Found in regional metamorphic terrains, particularly in areas that experienced greenschist facies metamorphism of mafic protoliths. Common in orogenic belts and ancient shield areas.
Key Facts
- Hardness: 5.5-6 on Mohs scale (for actinolite mineral)
- Specific Gravity: 3.0-3.2 (for actinolite mineral)
- Crystal System: Monoclinic (for actinolite mineral)
- Color: Green, grayish-green, dark green
- Luster: Silky, vitreous, dull
- Transparency: Translucent to opaque
- Fracture: Uneven to splintery
- Cleavage: Perfect on {110} at 56 and 124 (for actinolite mineral)
- Composition: Hydrous calcium magnesium iron silicate (Ca2(Mg,Fe)5Si8O22(OH)2)
Quick Check
- Color: Green to grayish-green
- Luster: Silky to vitreous
- Streak: White to pale green
Physical Characteristics
- Crystal Habit: Acicular, prismatic, fibrous, bladed (for actinolite mineral)
- Cleavage Type: Perfect prismatic (for actinolite mineral)
- Fracture Type: Uneven to splintery
- Tenacity: Brittle (for non-asbestiform actinolite); flexible (for asbestiform actinolite)
- Luster Type: Silky to vitreous
Formation
Actinolite schist forms under regional metamorphism of mafic igneous rocks (like basalt, gabbro) or magnesium-rich sedimentary rocks (like dolomitic shales) under greenschist facies conditions. This involves temperatures typically ranging from 300C to 500C and pressures from 2 to 10 kilobars. The presence of water is crucial for the hydration reactions that form actinolite from pyroxenes and amphiboles.
Usage
Actinolite schist itself has limited direct industrial uses due to its friable nature and the presence of fibrous actinolite. However, it can be a host rock for certain ore deposits (e.g., gold, copper). Historically, some varieties containing asbestiform actinolite were used in insulation and construction, but this practice has largely ceased due to health concerns. Fine-grained, compact varieties can be used as decorative stone or for carvings.
Age Distribution
Precambrian to Cenozoic, depending on the metamorphic event.
Where to Find
Appalachian Mountains, USA
Extensive occurrences in metamorphic belts, particularly in New England and the Piedmont region.
Alps, Europe
Common in the metamorphic core complexes of the European Alps, resulting from Alpine orogeny.
Fennoscandian Shield, Scandinavia
Widespread in Precambrian metamorphic terranes.
Western Australia
Found in various Archaean greenstone belts.
New Zealand
Present in metamorphic terrains, particularly in the Southern Alps.
Finding Tips
Look for Green Coloration
The most immediate indicator is the characteristic green color, often with a fibrous or silky sheen.
Check for Foliation
Examine the rock for a distinct planar fabric (schistosity) where minerals are aligned.
Identify Needle-like Crystals
With a hand lens, look for elongated, often needle-like or blade-like green crystals of actinolite.
Consider Geological Context
Actinolite schist is typically found in metamorphic zones, especially those associated with ancient oceanic crust or mafic intrusions.
Safety Precaution: Asbestiform Varieties
Be aware that some actinolite can occur in an asbestiform habit (fibrous, flexible, separable fibers). If you suspect asbestiform actinolite, do not disturb the rock, avoid inhalation of dust, and handle with extreme caution. Asbestiform actinolite is a known carcinogen. Non-asbestiform actinolite is generally safe to handle, but always minimize dust inhalation when breaking or cutting any rock.
Similar Rocks
Chlorite Schist
Chlorite Schist
Also known as: Chlorite-rich Schist
Greenschist
Greenschist
Also known as: Greenstone
Amphibolite
Amphibolite
Also known as: Hornblende Schist
Scientific Classification
- Mineral Class
- Silicate
- Group
- Amphibole Group, Clinopyroxene Subgroup
- Crystal System
- Monoclinic
- Chemical Formula
- Ca2(Mg,Fe)5Si8O22(OH)2
- Composition
- Calcium magnesium iron silicate hydroxide
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